950 MICROBIAL DISEASES OF PLANTS 



ville wilt than the large-leaf kinds. Resistance of this sort, which ap- 

 pears to be nothing other than a natural, inborn quality, may be des- 

 ignated as natural immunity, and it is immunity of this kind which 

 plant breeding for disease resistance has secured. A good illustration 

 of this is to be found in the wilt-resistant water melon of the Carolinas, 

 which is the result of crossing a naturally susceptible water melon with a 

 naturally resistant citron. 



Acquired immunity in the plant world is a field yet to be explored. 

 Cases have been cited in which active immunity appears to have followed 

 the disease, but these are extremely rare and the evidence is very 

 questionable. Passive immunity, at the present time, is unknown. 



